>Rich:
>
>>Does it make sense that the IMD products generated by a grid-driven
>>4-250A would DECREASE as the grid is driven from the AB1 region of zero
>>grid current into the AB2 region of grid current.? If this were true,
>>then it would be "EXACTLY correct" that the G2DAF circuit is pristine
>>clean. :)
>
>Have a look in The care and Feeding of Power Grid tubes: there's a set
>of curves of IMD against input as you get into grid current.
page number?
>Provided that the grid (and screen, if applicable)
If applicable? Have you ever seen a triode Class AB1 HF amplifier?
>stay at constant
>volts, and that the increased loading caused by grid current doesn't
>screw up the cleanliness of the driver, why should grid current cause
>distortion?
Study the constant current curves for grid driven configuration and the
answer will be revealed to you.
>....... there have been lots of Class AB AF amplifiers built
>where the tubes ran grid current............they needed a low impedance
>supply so that the drive volts didn't sag, if distortion was to be
>avoided.
This is true for Class AB2 push-pull AF amplifiers. Do you believe that
Class AB2 push-pull amplifiers and Class AB1 single-ended amplifiers have
identical rules and characteristics?
>
>After all, 3-500Z s in grounded grid draw grid current, without being
>disastrous on linearity.
Are we talking about cathode-driven Class AB2 or are we talking about
grid-driven Class AB1?
>I suspect that the G2DAF problems came from
>poor drive regulation, insufficient screen volts, and being touchy -
>Dick Thornley certainly produced some good looking pictures of two tone
>tests,
Yea, verily. During a 2-tone test the screen V was constant. During
voice modulation the screen V varied widely.
>and I seem to remember some guys at the Marconi Company getting
>good IMD on the spectrum analyser using that circuit. Somehow, I was
>never too impressed - seemed to me to rely too heavily on getting the
>ratios of voltages, drive etc just right.
Good IMD relied on using only constant level 2-tone modulation.
With voice modulation the total IMD was around 21db down..
cheers, Peter
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|